1. Field of the Invention
The present invention relates generally to a method for forming a coating layer on a cylindrical substrate using a high-frequency plasma chemical deposition technique and, more particularly, to a method and apparatus for simultaneously forming uniform coating layers on a large number of cylindrical substrates using multiple pairs of power supplied electrodes and ground electrodes which are disposed in a formation for achieving uniform synthesis of the coating layers.
2. Description of the Related Art
Recently, intensive researches have been made in order to improve the abrasion resistance and lubricity of a VCR head drum. For example, in the Report of Korean Ministry of Science and Technology BSU1434-4943-1, a method for increasing the abrasion resistance and lubricity of a head drum while preventing the damage of VCR tape had been disclosed. The technique for synthesizing diamond-like carbon film is generally divided into 4 categories: plasma chemical deposition, sputtering, ion beam synthesis, and laser ablation.
According to an article published in the Transaction of Korean Metallurgical Society, 6(4), 345(1993) by K. R. Lee and K. Y. Eun, the plasma chemical deposition method has a number of practical advantages. (1) This method is adapted for coating a substrate having complex configurations. (2) This method uses simple apparatuses which do not require experience for its operation. (3) This method reduces the coating cost because it does not require a high degree of vacuum and uses methane gas as the main carbon source.
However, according to the aforementioned article, a diamond-like carbon film has optical transparency, which causes the interference color generated by the coated film to differ depending on the thickness of the film. Therefore, it is very important to maintain a uniform synthesis rate along the circumference of a head drum for producing coated head drums having the same color.
A technique for obtaining an uniform coating layer along the circumference of a head drum is described in Korean Patent Application No. 94-18059 invented by the inventors of the present application and assigned to the assignee of the present application. In this technique, a diamond-like carbon film is synthesized using cylindrical plasma generated around the head drum by applying high-frequency electrical power to the stacked cylindrical head drums and disposing an annular ground electrode in the form of a concentric circle surrounding the stacked head drums. By using this synthesizing technique, a very uniform plasma can be generated along the circumference of the head drums, which allows the uniform synthesis of coating layers without requiring additional manipulation such as rotating the head drums.
FIG. 1 shows an embodiment of an high-frequency plasma chemical deposition apparatus 10 using only one pair of electrodes. This embodiment is also disclosed in '059 Korean Patent Application. This synthesizing apparatus generally comprises a reactor 10-1, a power supply system 10-2, a gas supply system 10-3, and a vacuum system 10-4. The reactor 10-1, which is in a vacuum state, comprises an electrode 1a disposed at the center of the reactor for supplying power and a conductive workpiece support 1b on which VCR head drums are stacked. The workpiece support 1b is attached to the electrode 1a to form power supply electrode 1. An annular shield type ground electrode 3 is disposed in the form of a concentric circle spaced 1-10 cm from the cylindrical surface of the head drums. To the reactor 10-1, the following systems are connected: a power supply system 10-2 for supplying power to the electrode 1a and for monitoring the electrical characteristics thereof; a gas supply system 10-3 for providing the source material in the gaseous state for the synthesis; and a vacuum system for maintaining a vacuum state and for controlling the pressure in the reactor. Both the workpiece support 1b and the head drum are made of a conductive material. Thus, when the electrical power is supplied to power supply electrode 1 through power supply system 10-2 and the source gas is introduced into reactor 10-1, the plasma of the source gas is generated between ground electrode 3 and the surface of the stacked head drums. The source gas decomposed by the plasma is deposited on the head drum substrate in the form of diamond-like carbon film.
On the other hand, to coat a large number of head drums simultaneously using a synthesizing apparatus having one pair of electrodes, more number of head drums have to be stacked and, therefore, the length of electrode has to be extended. The extension of the electrode causes a number of problems as follows: (1) The temperature of the electrode increases sharply owing to the large quantity of electrical power applied thereto, which causes problems in the vacuum sealing and results in the deformation of the stacked head drums adjacent to the electrode; (2) Very stiff temperature gradient generates between the stacked head drums because the workpiece support cannot be cooled by the circulation of coolant. Therefore, the property of the synthesized film greatly differs according to the location; (3) The composition of plasma varies because the uniform generation of plasma along the longitudinal direction of electrode becomes more difficult and the difference in the distribution of source gas becomes great along the length of electrode; (4) In order to form an uniform plasma in the circumferential direction, the space between the ground electrode and the surface of the head drum must be uniform. But, as the electrode becomes longer, the fabrication of a ground electrode having precise circular cross section becomes more difficult; (5) The long length of the workpiece support interferes the mounting of the head drums.
When a large number of head drums is simultaneously coated by a synthesizing apparatus having one pair of electrodes, damage to the head drum may occur which results in difficulty in obtaining a coating layer having uniform thickness because of the non-uniform synthesizing rate. These problems hinder the commercial use of this apparatus.
Therefore, a need exists in the art for an apparatus that can coat a large number of cylindrical substrates simultaneously with uniform coating rate.